Separating documents in a stream

[John Tigue]

Richard Anderson (rja at arpsolutions.demon.co.uk) wrote:
> Provided the xml pi is _always_ used that can be used 
> as the separator and seems the cleanest approach to me.
> 
> ----- Original Message -----
> From: David Megginson <david at megginson.com>
> > kragen at pobox.com (Kragen Sitaker) writes:
> >
> > > One protocol I remember reading about uses ]]>]]> to 
> > > separate XML documents, since that's a sequence that 
> > > can't occur inside a legal XML document.
> >
> > True, but ^L (formfeed) is not allowed within an XML 
> > document either -- it's still my favourite as a separator.

I'd consider using multipart MIME to solve the problem 
of seperating documents in a stream.

Here's the spec:
IETF RFC 1341
Borenstein, N. and Freed, N., 
"MIME (Multipurpose Internet Mail Extensions): Mechanisms for
Specifying and Describing the Format of Internet Message Bodies"
http://www.ietf.org/rfc/rfc1341.txt

Multiparting is well defined, well tested, and there exists free 
compliant software. Also by using technology which is orthogonal 
to XML, implementation code can be re-used.

Here's an example of what multipart looks like:

MIME-Version: 1.0
Content-Type: multipart/digest; boundary="---- next message ----"

------ next message ----

...document 1 goes here ...

------ next message ----

...document 2 goes here...

------ next message ------



There's even a sub-type 'multipart/related' which works well for 
aggregate documents with interlinked parts and a defined root
entity (for example, for HTML email the root is the HTML document 
and it links to the non-root images). multipart/related maintains
the internal links during transfer. XML document are a good
candidate for multipart/related.

Here's the multipart/related spec:
IETF RFC 2112
Levinson, E.,
"The MIME Multipart/Related Content-type"
http://www.ietf.org/rfc/rfc2112.txt

Here's a modified example from the spec. In this example there
are 3 parts: 1 HTML-like doc and 2 JPEGs.

Content-Type: Multipart/Related; 
     boundary=example-2; 
     start="950118.AEBH at XIson.com"
     type="text/xml"

--example-2
Content-Type: text/xml; charset=iso-8859-1;
     declaration="950118.AEB0 at XIson.com"
Content-ID: 950118.AEBH at XIson.com
Content-Description: Document

<doc>
This picture was taken by an automatic camera mounted ...
<image file="cid:950118.AECB at XIson.com" />
<para>
 Now this is an enlargement of the area ...
<image file="cid:950118:AFDH at XIson.com" />
</doc>
--example-2
Content-Type: image/jpeg
Content-ID: 950118.AFDH at XIson.com
Content-Transfer-Encoding: BASE64
Content-Description: Picture A

[encoded jpeg image]
--example-2
Content-Type: image/jpeg
Content-ID: 950118.AECB at XIson.com
Content-Transfer-Encoding: BASE64
Content-Description: Picture B

[encoded jpeg image]
--example-2--



-John




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